1. Field of the Invention
The present invention relates to a thin film deposition method, and, in particular, to the process for depositing a thin film on a wafer in a vacuum chamber or vessel where any reactant gas is supplied and a thin film is deposited on the wafer under the pressure range in which the molecules in the supplied gas will have no reaction interaction upon each other in their vapor phase.
2. Description of the Prior Art
There is a conventional thin film deposition method which is known as the "gas source epitaxial growth" process. In this process, a vacuum chamber or vessel has its inner wall covered with a shroud, and is internally placed under the liquid nitrogen, allowing the shroud to be cooled to the liquid nitrogen temperature. The molecules in the reactant gas from its source are diffusing as it is introducing into the vacuum chamber, and the major part of those molecules may be attracted toward the inner wall and absorbed on the shroud. During the epitaxial growth process, therefore, only the small remaining part of the molecules in the reactant gas can be flying across the space directly from the source, such as gas nozzle and other gas sources, onto the wafer surface. In this specification, this part of the molecules will be referred to hereinafter as "primary molecules".
The primary molecules which are traveling from the gas source onto the wafer surface may usually have a sticking coefficient of less than 10%. It is noted that the gaseous molecules from the source are diffusing according to the cosine law just after they have been introduced into the vacuum chamber, and the proportion of the gaseous molecules that can be flying onto the wafer surface is equal to 10% to 20% of the total gaseous molecules that have been supplied from the source.
It may be understood from the above description that the conventional thin film deposition method cannot utilize the reactant gas effectively during the epitaxial growth process, whose utilization is only equal to several percent of the total reactant gas.
It has been described that the reactant gas supplied from its source is diffusing according to the cosine law as described above. Similarly, when a thin film is being deposited on the wafer surface during the epitaxial growth process, it may also be influenced by the above cosine law in such a way that the film thickness distribution is affected. It is thus difficult to provide the uniform film thickness distribution across the wafer.